Optical coherence tomography (OCT) is a high-resolution medical and biological imaging technology. OCT utilizes low-coherence interferometry (LCI) to perform optical ranging within biological tissues. OCT has already previously been demonstrated for being a high resolution real time imaging modality that can provide near-histological information in other clinical applications such as ophthalmology, cardiology, and digestive disease. In use, OCT detects the reflections of low-coherence light, and cross-sectional imaging may be performed by measuring the backscattered intensity of light from structures in tissue. This imaging technique is attractive for medical imaging because it permits the imaging of tissue microstructure in situ. In situ imaging with OCT provides micron-scale imaging resolution without the need for excision and histological processing. OCT has been used in ophthalmology for high-resolution tomographic imaging of the retina and anterior eye. Recently, the technique has been applied for imaging a wide range of nontransparent tissues to investigate applications in tissues studies and medical applications in gastroenterology, urology, and neurosurgery.
OCT measures cross-sectional tomographic images in tissue and is similar to ultrasound B-mode imaging except that it uses light waves rather than sound. OCT also differs from ultrasound in that the detection in OCT is based on interferometry. In ultrasound, the time the ultrasound pulse takes to travel to a surface and be reflected back can be measured by an electronic clock. However, this is not possible with optical techniques because of the high speeds associated with the propagation of light. This limitation is overcome with the use of a reference light path and interferometry. A detailed presentation of the principles of operation of OCT and factors that govern its performance have been previously published. (See Huang D, Swanson, Lin C P, Schuman J S, Stinson W G, Chang W, Hee M R, Flotte T, Gregory K, Puliafito C A, Fujimot J G. Optical coherence tomography. Science. 1991:254:1178-1181; and Swanson E A, Izatt J, Hee M R, Huang D, Lin C P, Schuman J S, Puliafito C A, Fujimoto J G. In vivo retinal imaging by optical coherence tomography. Optics Lett. 1993; 18:1864-1866; both of which are herein incorporated by reference.)
OCT systems may use fiber optics and a compact diode light source similar to those used in compact disc players. Precision distance measurements may be performed by Michelson-type interferometry. In this case, light from the source is split by an optical fiber splitter, which functions as an interferometer. One of the fibers directs light to the tissue and the other to a moving reference mirror, in the case of time-domain OCT. The distal end of the optical fiber can be interfaced to a catheter. In time-domain OCT, the position of the reference mirror is precisely controlled by the system electronics. The light signal reflected from the tissue is recombined with the signal reflected from the mirror. Interference between the light reflected from the tissue and the light reflected from the reference mirror occurs only when the two path lengths are matched to within the coherence length of the light source. This allows precise (micron scale) determination of the distance within the sample from which the light was reflected.
OCT therefore measures the intensity of backscattered (reflected) light from within the tissue, plotted as a function of depth. A cross-sectional image is produced in a manner similar to radar by recording axial reflectance profiles while the transverse position of the optical beam on the tissue specimen is scanned. The image is displayed either in gray scale or false color in order to differentiate tissue microstructure.
Spectroscopic optical coherence tomography (SOCT) is an extension of OCT that can differentiate between different types of tissue. In addition to the normal OCT measurement of the intensity of light backscattered from the sample, SOCT measures the spectral absorption and scattering data from the tissue. Tissue structure can be resolved based on local optical densities, ignoring the frequency dependent changes. SOCT resolves both the amplitude, which contains the scattering and refractive of index information, and the frequency, which contains spectroscopic molecular composition information based on the absorption and scattering properties.
Contrast agents may be used to improve the specificity and targeted tissue visualization of images obtained from an imaging technique, including OCT. Conventional contrast agents serve to increase the intensity of backscattered light. For example, air-filled micro-bubbles and engineering microspheres may be introduced into tissue to increase the backscattering from tissue. In another example, a molecular contrast agent can be used in a pump-probe technique to change the absorption properties of the light.
Currently, it is difficult for surgeons to differentiate between normal and tumor tissue, for example at tumor margins, at the cellular level. Tumor margins may include a tumor mass, which is a mass of abnormal cells, or tumor cells, which are abnormal cells, without a tumor mass. Tumor margins may also include both a tumor mass and tumor cells, which may or may not surround the tumor mass. Tumor margins are classified as either positive—meaning diseased or cancer cells are found on or near the surface of the excised tissue specimen, close—meaning diseased or cancer cells are found within a few mm of the surface, or negative—meaning no diseased or cancer cells are found. These dimensions are the most commonly used dimensions and serve as guidelines in the definition of positive, close, or negative margins. Once the tissue specimen is excised, it is then typically sent to a radiology department for imaging using plain-film X-rays in order to receive a gross confirmation of a wide enough clean margin around the lesion, particularly if metal localization wires or beads were placed in or near the tumor site prior to surgery. The gold standard is to send the tissue specimen to the pathology department where the pathologists will first perform a gross examination of the margin and subsequently evaluate stained tissue sections using light microscopy to view them. Although these are the most common methods used by surgeons to determine whether enough tissue has been removed from the patient during a procedure such as surgery, all diagnostic decisions on a positive, close, or negative tumor margins rely on traditional haematoxylin and eosin, or immunohistochemical staining of a paraffin embedded specimen and evaluation by a pathologist, which can take from hours to days, to determine the presence of cancer cells.
With an increased number of cases as a result of earlier detection or screening of cancer, tumors have become smaller, frequently are non-palpable, and often have unclear demarcations delineating tumor tissue from normal tissue. Therefore, without a real-time in vivo method for microscopic analysis of the tumor margin, surgeons must rely on their own judgment for taking a wide enough margin of normal tissue around a tumor to ensure a negative margin, or wait until the radiology department and/or the pathology department weighs in on the status of the tumor margin.
The current rate of positive margins following solid tumor resection can be significantly high. Research studies have found that the positive-margin rate for breast lumpectomy specimens is as high 64% following the first resection, and fall to only 21% after the third resection. If positive margins are identified while the patient is still in surgery, additional tissue may be taken out. However, if the positive margins are not identified until the final pathological assessment (which often takes at least 24-48 hours post-surgery), the patient will have to return to the hospital for a second surgical procedure to remove additional tissue.
Prior to surgical resection, a tissue specimen is often needed to make a pathological diagnosis and direct treatment options. To obtain tissue, needle-biopsy procedures are frequently performed where a needle is inserted transcutaneously and passed to the site of the suspected tumor mass. Often, an external imaging system is used to facilitate placement of the needle, such as X-ray stereotactic localization, X-ray CT, MRI, or ultrasound. The reliance on these imaging modalities has increased as the number of smaller, non-palpable lesions or masses has increased.
Guiding the tip of the biopsy needle (frequently <1 mm in size) to the correct location at the site of an abnormal lesion (frequently <1 cm) or mass is highly problematic due to lack of operator experience, patient body habitus, mass location, and the imaging field-of-view provided by the external imaging system. Additionally, the lack of being able to localize the lesion in a third dimension of view makes it difficult to find the lesion. Having an evaluation technique at the end of the biopsy needle would prove to be highly useful. Frequently, non-diagnostic samples are obtained from needle-biopsy procedures, which implies that despite an abnormal finding on imaging or exam, only normal tissue is extracted in the needle-biopsy. The non-diagnostic sampling rates can be quite high. For breast masses less than 1 cm in size, the rate is approximately 10-20%. For lung nodules less than 1 cm in size, the rate is as high as 50%. Subsequently, patients require more invasive and extensive open surgical procedures in order to resect the suspicious mass and obtain a diagnosis.
It would be highly desirable to shift the high-resolution microscopic analysis of tumor specimens out of the remote pathology lab and to the point-of-care, or in vivo, where diagnosis and treatment decisions can be made in real-time.
Surgeons do not currently have a standard, reliable, method for assessing lymph nodes interoperatively to determine if they are tumor-bearing. Lymph nodes are the major points of drainage for foreign particles introduced into the human body. Cancer cells that have migrated away from the primary tumor are drained into not only the blood circulation, but also into the lymphatic system and into the lymph nodes. At these sites, the body produces an immune response to combat the cancer cells. This initial interaction produces a reactive lymph node. However, as the cancer cells become more virulent and outgrow or outpace the immune response, they could potentially travel to other organ systems via the lymphatic system and establish secondary or metastatic tumors. Prior to this latter stage, the lymph node is deemed a tumor-bearing node and is often an earlier sign of the potential formation of a metastatic tumor. Currently, it is often necessary to remove a lymph node and evaluate the stained sections under light microscopy in order to determine if the lymph node is tumor-bearing. However, in doing so, pathologists take step-wise sections through the lymph node and prepare these for white-light microscopy assessment. This sectioning protocol will only examine approximately 5% of the entire lymph node, missing a large percentage of the node.
It would be desirable to perform real-time in vivo assessment of a lymph node prior to its resection. This would lead to a reduced number of non-diagnostic lymph nodes being removed and reduced associated complications such as lymphedema, which is the accumulation of lymph fluid at the affected site due to the disruption of the lymphatic network by the removal of lymph nodes.
A number of laboratories have worked toward the detection of cancer, such as breast cancer, using endogenous, or native, optical contrast. Many of the techniques used to exploit this contrast rely upon differences in the spectroscopic response of tissue. Raman spectroscopy, for example, is a nonlinear process that can be used to identify optical signatures due to the chemical composition of tissue. Studies have shown that this technique is effective at distinguishing between normal and diseased tissue in surgical specimens. Similarly, spectral attenuation signatures are well known to vary between healthy and diseased tissue, but these measurements are generally used for evaluation of the intact breast. These techniques have some issues. First, they are not based on the structural properties of the tissue, but rather the chemical signatures, which do not provide information overlayed on the tissue structure, making it harder to localize the tumor or the abnormal tissue depth wise. Second, they are not imaging modalities, per se; they often provide data at a single probing point, not over a region. Third, they do not provide enough cross-sectional depth-wise imaging. Fourth, they are not real-time modalities, but generally require long acquisition time in order to generate sufficient signal for analysis.
Fluorescence-base techniques have also been investigated for the detection of cancer. These methods make use of dyes that are often administered intravenously or topically to the surgical tumor site. The dyes aggregate in the region of the tumor, and the targeted tissue fluoresces when illuminated with an appropriate light source. This technique yields good detection over a large area and could potentially be augmented with microscopic equipment in the operating room. However, these techniques require the use of drug/probe administration, and are limited to surface viewing. Additionally, this method is not suitable for needle-biopsy guidance.
Frozen-section histology has also been used for the detection of cancer during surgical procedures. The problems with frozen-section histology include that it is very time consuming, often yields poor quality results, and usually post-operative histology is still performed. Touch-prep cytology has also been tried for cancer detection. Touch-prep cytology requires a tumor mass margin to be touched to microscope slides and then these slides are viewed under a microscope for tumor cells.
Real-time PCR is another method for detection of cancer. The problem with real-time PCR is that it destroys the tissue sample in an effort to detect abnormal DNA or tumor-identifying material. Various targeted agents and dyes applied topically or intravenously have also been tried for cancer detection. The problem with theses targeted agents and dyes is that they require administration to the patient, either before or during the procedure, their targeting (localization) to diseased cells or tumors is often insensitive and/or non-specific, and the optical systems used to detect these agents lack a high enough magnification and resolution to be able to detect all of the cancer, and individual cancer cells. These agents and dyes are also only viewed at the surface of the tissue, not below the surface.